1. Field of the Invention
The present invention relates to a method for contactless determination of the thickness of a web of material, for example a fibrous material web with a sensor arrangement including at least two optical measuring units between which the material web can be guided and which respectively include a measurement plate on the side facing the material web. The optical measuring units which are arranged on opposite sides of the material web are used to measure the distance of the measuring units from the material web and an evaluation unit is used to determine the thickness of the material web from the determined distances between the optical measurement units and the material web and the distance between the optical measurement units arranged on opposite sides of the material web. The fibrous web may in particular be a paper or cardboard web.
2. Description of the Related Art
Optical thickness sensors for contactless determination of the thickness of a material web, especially a fibrous web are known from publications EP 1 855 082 A1 and EP 1 855 083 A1.
In contactless measuring of paper thickness the distance between the optical measuring units, or respectively measurement plates which are arranged on opposite sides of the web is to be determined on the one hand, and on the other hand the distances between these measuring units or respectively measurement plates and the web are to be determined, whereby the distances between the 3 measuring units and the web are determined by means of an optical measurement. The optical measuring units which are arranged on opposite sides of the material web must be positioned in precisely the same optical axis in order to eliminate faulty measurements due to a paper web which is not progressing perpendicular to the optical axis.
In particular, due to the air build-up of the moving web tilting between the optical measuring units or measuring heads arranged on opposite sides of the web can occur, resulting in an erroneous thickness value of the web.
Such tilting between an upper and a lower optical measuring unit or respectively measurement plates 12, 14 caused for example by the air build-up of the moving web or respectively fibrous web 10 can be seen in FIG. 1 which illustrates a schematic depiction of a conventional device for contactless determination of the thickness, wherein only one optical sensor 16 or respectively 18 is allocated to a respective measuring unit. In a conventional device of this type tilting of the measurement plates 12, 14 and a possible offset between the upper and the lower optical measuring unit or respectively their measurement plates 12, 14 cannot be compensated. Therefore, in a conventional device of this type with only one path of rays on a respective web, an inaccurate measurement results.
If, in addition, an offset occurs between the upper and lower sensor it would result in that an additional error occurs in measuring the material thickness. An offset may be caused by a misalignment between upper and lower sensor carriage or through tilting of the sensing heads.
What is needed in the art is an improved method as well as an improved device, with which faulty measurements are appropriately compensated and with which a more precise measurement is achieved.